[TowerTalk] Rooftop vertical grounding [SUMMARY]

[Guy Olinger, K2AV]

I think Jeff should continue his good plan as he has specified, and NOT
skimp on the radials.

Providing a low loss current sink for the energy from the coax shield
(the other half of the dipole, as you say) is ONLY ONE of the functions
of the radial screen underneath for the Butternut.

A second function is to provide a conductive shield that keeps the near
fields from penetrating and inducing current in lossy ground directly
underneath. This is mostly done near to the center of the radial field.

The third function is to minimize reflective losses moving away from the
center of the screen. These have to do with Fresnel losses, the Brewster
effect, the pseudo-Brewster angle, etc, and are critical for obtaining
good low-angle radiation performance.

Even a Gap Titan, which is a shortened multiband vertical dipole, and
needs no radials as a current sink, benefits from a *dense* 65 foot
radius ground screen underneath.

For raised radials to perform ALL THREE functions, sixty raised radials
have been reported as a magic number for top signal reports. ON4UN
recommends a ground screen below anything less than twenty raised
radials.

The principles above are extensively discussed in his excellent book
"Low Band DXing - 3rd Ed", p.p. 9-9 ff.

Some vertical plans, in order to keep RF charged conductors out of
reach, have put the vertical base up 7 or 8 feet on a ten foot treated
4x4. Four raised radials up seven or eight feet AND a dense radial
ground screen centered on the base of the 4x4 plus ground rod at the 4x4
and ground wire up the 4x4 all tied together. The 4 radials provided the
current sink. The ground screen took care of functions two and three
above.

That scheme was used by my Elmer back in 1958, out in his back yard, for
his 40 meter ground plane setup. At the time he was the chief engineer
at AM station WCTT in Corbin, KY, and was the builder of the low end of
the band (570?) station and antenna farm outside town lovingly known
among the local radio population as "The Swamp". Turns out after all
these years, he knew what he was doing.

- - . . .   . . . - -     .   . . .     - - .   . - . .

73, Guy
k2av@contesting.com
Apex, NC, USA

----- Original Message -----
From: Tom Osborne <w7why@harborside.com>
To: <jrp@dimensional.com>; TowerTalk <towertalk@contesting.com>
Sent: Saturday, February 12, 2000 2:49 AM
Subject: Re: [TowerTalk] Rooftop vertical grounding [SUMMARY]


>
>
>
> berferd wrote:
>
> > So instead I'm going to ground mount the Butternut in the >middle of
my backyard with a buried radial system.
> >
>
> Hi Jeff
>
> If possible, the antenna would work a lot better and require a
> lot fewer radials if you could mount it 8 or 10 feet in the air.
> Verticals with raised radials work much better that buried
> radials.  The purpose of the radials is to transfer the power
> back to the feed point of the antenna for efficiency.  When the
> radials are buried, it is harder for the RF to get to them for
> the return path.  Think of the radial system as the other half of
> a dipole antenna.  I have 3 phased verticals here and a 160 meter
> inverted "L" antenna with many many radials.  Mine are just
> laying on top of the ground as there is nothing the back yard
> except trees and brush.  With a ground mounted radial system, it
> takes about 60 radial to make it play, but with raised verticals
> and radials, you can get by with as little as 4 radials and make
> the antenna play.  The radials are not on the antenna for
> grounding purposes but to to provide a path for the return the RF
> back to the feed point.  73
> Tom W7WHY
>
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>


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